In operation of a PFBC (Pressurized Fluidized Bed Combustion) plant, a varying part of the tubes which are to generate steam for a steam turbine in the plant will be covered by the fluidized bed in dependence on the degree of load. In case of full-load operation, the tubes are completely covered, and in case of partial-load operation they are partially covered. One problem in this connection is that the non-covered tube portions cool the combustion gases which leave the bed at a temperature of about 850.degree. C. A reduced temperature of the gases to the gas turbine entails, among other things, a deteriorated efficiency of the plant, favorable conditions for NO.sub.x reduction, and an increased CO content in the waste gases from the plant.